Acids, Bases, and Salts

If I have time, i will edit this lecture. For students at USA starting in Spr 18, please note:

Ignore first 14.5 min. Ignore 28.26-44.15. Ignore 46.08-51.40. Watch from 51.41 to the end.

As a review from Ch 5, the list of strong acids and bases are the most important and the most common acids and bases. Know their formulas. Remember that the word "strong" means complete ionization.

The first and most common definition especially in a biological sense is the Arrhenius system. This system is confined to aqueous solutions of acids and bases but since we live in a pretty watery world, this is a convenient and applicable system. Acids increase the concentration of H+ in aqueous solution. Remember that the hydrogen ion does not really exist alone and if there is water around (which there will be for Arrhenius acids and bases), it will be the hydronium ion, H3O+, that will be produced by Arrhenius acids. Bases increase the concentration of OH- ions in aqueous solutions.

When an acid and a base combine, the product is most often water and a salt. Instead of forming a precipitate, molecular water is formed which drives these reactions forward. The process for writing the reaction is the same. As the picture below represents, acid + base reactions are often called neutralization reactions which can be misleading. Not all acid + base reactions will end with a neutral pH. You will learn later that the pH at the endpoint (titration term) depends on the identity of the salt.

HCl (aq) + NaOH (aq) → NaCl(aq) + H2O

In pure water, hydronium and hydroxide concentrations are equal and we call pure water neutral. When [H3O+] > [OH-] then the solution (now water is not pure ) is said to be acidic and when [H3O+] < [OH-], the solution is said to be basic. The concentration of [H3O+] or [OH-] in an acidic or basic solution will be determined by the stoichiometry of the acid or base. For example, if the concentration of HCl is 1 x 10-3 M, then the hydronium ion concentration in this solution will also be 1 x 10-3 M. If you need the hydroxide ion concentration for the same solution, use the Kw relationship: Kw = [H3O+][OH-] and the hydroxide concentration for this example is

1 x 10-14/ 1 x 10-3 = 1 x 10-11 M. See second chart below (memorize this one). It is important to realize, however, that this simple relationship between acid/base concentration and hydronium/hydroxide concentrations only works for STRONG ACIDS AND BASES. Because concentrations like 1 x 10-8 and so on are hard to write and conceptualize for many, a log of the Kw expression gives us an easier scale to use. The important thing to realize is that because the pH scale is logarithmic, every change in value of 1 on the pH scale really represents a change of 10 in concentration.

Chemistry is a subject that must be practiced everyday if possible. Work through the lecture examples stopping the video clips and then restarting to check yourself. Take advantage of the practice in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Email me with questions!!

Email: oliver@southalabama.edu
Location: University of South Alabama, Mobile, AL, United States
Phone: (251)405-4504